Update -- what seems to make the difference for the editor adjusting the selection is the right-click spell checking kicking for the nonsense text I was using to test. If I use ALL real words in mixed case ...
Code:
<p>original = cat doG bird hoUSE Car boat TRAIN</p>
<p>after = CAT DOG BIRD HOUSE CAR BOAT TRAIN</p>
... then the Change Case works
But what was/is confusing me was the sensitivity of the editor's changing the selection depending on the cursor position after selecting the test.
Below when I selected the text between the tags, and right click on a real word (e.g. doG) all text remains selected and no spell check correction pops up and Change Case happens as expected. However if I select a word not in the dictionary (even though it should be

) only that word is selected since the spell check popup seems to take precedence and Change Case is applied to only the single selected word
Code:
<p>after = cat doG bird hoUSE Car boat TRAIN KovidGoyalIsGREAT</p>